A light way to make smaller sensors

Tom Shelley reports on development work aimed at making more integrated optical sensing systems and bringing some manufacturing back from the Far East A combination of new technologies allows sensing systems based on fibre optics or even bench top spectrometers, to be entirely embodied in single chip level devices. As well as to pointing the way to even lower cost sensing systems for environmental monitoring and medical work, it enables the manufacture of complex opto-electronic communication technology devices by pick and place robots. It is therefore a technology combination that should at the same time improve doctor's office diagnostics, and bring a sizeable amount of advanced system manufacturing back from low wage economies to Britain. The Centre for Integrated Photonics, based on BT's Adastral science park at Martlesham Heath originally started out developing its integrated opto electronic devices for BT when it was heavily involved developing what became the optical fibre communication revolution. It then passed into the hands of Corning until that company returned to its core glass based businesses. Immediately prior to this decision, Corning invested a large amount in providing the Centre with new laboratories and state-of-the-art equipment. It is therefore to everyone's benefit that both the expertise and equipment were subsequently rescued by the East of England Development Agency with the agenda of turning it into a major bridge between UK university research and industry. The new sensing systems are derived from waveguide interferometers. Integrated optics reduced the size of such systems from bench top size to something that would sit in the palm of the hand. But, according the CIP's Graeme Maxwell, integrating a silicon oxide planar optical track into a device built on silicon allows 30 chips, each of which might incorporated as many as 20 such devices, to be manufactured on a single six inch silicon wafer. The wave guides are typically made by flame hydrolysis deposition in which a silica 'soot' is deposited in silicon, fused at high temperatures to form wave guides. Mach Zehnder and Sagnac interferometers have been made. The Mach Zehnder type incorporates a spiral optical track in one arm of the interferometer, light through which produces interference fringes when it interacts with light passing through a reference arm. The fringes move when the optical path length of the coil changes. The coil path can be doped or coated so as to interact with a chemical species being measured. Big technical advantages of using planar wave guides as opposed to fibre include the fact that they are much more rugged, and relatively unaffected by strain and temperature. On the other hand, sensing chips can be made to respond to strain in a very predictable way, demonstrated in university laboratories but yet to be commercially exploited, or similarly respond to temperature. Where temperature is an interfering parameter, it can also be measured separately and compensated for digitally. A Sagnac interferometer is even simpler in that it uses only a single loop of optical track, with the two beams of light passing through it in opposite directions, split by an optical coupler connecting the two sides of the loop at its base. Such devices made of optical fibre have in the past been used for current sensing. It is also possible to etch Bragg gratings onto optical paths, similar to the manner in which they are etched onto optical fibre to make demultiplexers. Asking CIP's David Smith whether any of these devices might be suitable for DNA characterisation elicited the reply, "We can't be specific but I can say we have been talking to various organisations about this." Digital compensation, signal conditioning or communicating electronics could also in theory be integrated onto the silicon substrate used to support the silica wave guide. There is also the possibility of integrating sensing with micro fluidics for 'lab on a chip' devices. Light emitting and detecting devices, on the other hand are best made in indium phosphide which can be attached to a silicon daughter board and made to fit into a precise location on the optical motherboard, etched out by micro-machining. Other suitable substrates for electro-active components studied which could be mounted on the motherboard include gallium arsenide at visible light frequencies and gallium nitride for frequencies at the blue end of the spectrum. Micro-manufacturing integrated systems for sensors and telecommunications, without need for rows of people at benches, usually in the Far East, manipulating fibre optic components under microscopes, is also one of the main goals of development work at CIP. As well as making mechanical structures by micro-machining silicon, the team has spent time working with SU-8, a polymer developed by IBM. This material can be coated onto a motherboard and photo-etched to form mechanical end stops to facilitate assembly. Pick and place robots are nowadays able to work to accuracies of down to 1 micron, but pushing against an end stop reduces this to a fraction of a micron. Maxwell describes making constructions from SU-8 as "Rather like working with 'Lego'" and their fabrication method as a "Poor man's LIGA" because it permits the making of high aspect ratio plastic structures without requiring the use of an X-ray synchrotron source. Constructions can be made 0.5mm high or more if necessary. Applications for the sensors are seen in water and atmospheric pollution monitoring and in medical work. They could also be the eventual winners in the race to produce sensors that can screen large numbers of people, some of whom might possibly be infected with SARS or other diseases, check cattle for possible foot and mouth and perform analyses in doctor's offices without patients having to visit hospitals. Smith says that CIP's largest present contract is to supply components to a group of universities led by University College, London, and including the universities of Cambridge, Essex and Imperial College. It also works with Advantage West Midlands and the South West Development Agency as well as various small and not so small companies. The Centre for Integrated Photonics David Smith at the Centre for Integrated Photonics Eureka says: Integrating optics with electronics is the next stage on from the revolution in going from macroscopic optics to fibre optics Pointers * Optical paths can be made in silicon oxide on silicon chip devices * Devices so made are much smaller, more robust, less sensitive to temperature and strain and potentially much cheaper than those based on optical fibres * Integration between devices based on different substrates can readily be achieved by micro machining or etching SU-8 polymer structures so as to permit assembly to the required standards of accuracy by pick and place robots